Poly(trimethylene therephthalate) continuous manufacturing process

ABSTRACT

This invention relates to a continuous process for production of poly(trimethylene terephthalate), wherein gaseous 1,3-propanediol by product resulting from the process is condensed in a condenser, and a portion of the condensed by-product is recycled to the condenser while anther portion is recycled back into the process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119 from ProvisionalApplication No. 60/752,318 (filed Dec. 21, 2005), the disclosure ofwhich is incorporated by reference herein for all purposes as if fullyset forth.

FIELD OF THE INVENTION

This invention relates to a continuous process for production ofpoly(trimethylene terephthalate), wherein gaseous 1,3-propanediolby-product resulting from the process is condensed in a condenser, and aportion of the condensed by-product is recycled to the condenser whileanother portion is recycled back into the process.

BACKGROUND OF THE INVENTION

Poly(trimethylene terephthalate) is produced by reaction of terephthalicacid (TPA) or dimethyl terephthalate (DMT) and excess 1,3-propanediol atelevated temperatures to obtain an esterification product. Thisesterification product is subjected to a precondensation, and then theprecondensation product is subjected to polycondensation to obtainpoly(trimethylene terephthalate).

In the poly(trimethylene terephthalate) process, excess 1,3-propanediolis removed by volatilization from the precondensation andpolycondensation stages. This volatilized by-product 1,3-propanediol isknown to contain several additional by-products, e.g., trimethyleneterephthalate cyclic dimer and poly(trimethylene terephthalate)oligomers as well as some carbonyl containing compounds. Furthermore, ifthe starting material for the process includes dimethyl terephthalate,there may even be small amounts of it found in the by-product1,3-propanediol. Recycling by-product 1,3-propanediol is desirable inorder to improve the efficiency and lower the costs of the process.

Recent experience in operation of continuous processes for producingpoly(trimethylene terephthalate), however, has shown that solidby-products in the liquid by-product 1,3-propanediol graduallyprecipitate on pipes, heat exchanger walls and spray nozzles, etc. Theprecipitates may cause fouling, which in turn results in lower1,3-propanediol recirculation flow rates and eventual poor spraycondenser operation. This buildup of solids in the recirculation systemleads to shortened operational life, increased maintenance frequencyand, consequently, higher costs due to increased downtime, maintenancecosts and lower overall product yields.

U.S. Pat. No. 6,353,062, U.S. Pat. No. 6,538,076, US 2003-0220465A1 andUS2005-0165178 A1 disclose continuous processes for preparingpoly(trimethylene terephthalate) by polymerization ofbis-3-hydroxypropyl terephthalate. Excess 1,3-propanediol vapors areremoved from the process stream and condensed by means of a spraycondenser where they are cooled by being sprayed with condensed1,3-propanediol that has been cooled to less than 60° C., and preferablyless than 50° C. The condensed 1,3-propanediol flows into a hotwellwhere it is combined with additional 1,3-propanediol. A portion of theliquid in the hotwell is pumped through a cooler (i.e., a heatexchanger) to the top of the condenser for use as the condensing spray.None of these documents discloses recycle of excess 1,3-propanediol.

U.S. Pat. No. 6,277,947 and U.S. Pat. No. 6,326,456 disclose processesfor producing poly(trimethylene terephthalate) by esterification ofterephthalic acid with trimethylene glycol in the presence of acatalytic titanium compound, followed by precondensation andpolycondensation. The esterification is effected in at least two stages,where in the first stage a total molar ratio of trimethylene glycol toterephthalic acid of 1.15 to 2.5, a content of titanium of 0 to 40 ppm,a temperature of 240 to 275° C., and a pressure of 1 to 3.5 bar areused. In the at least one subsequent stage, the content of titanium isadjusted to be higher than in the initial stage by 35 to 110 ppm. Thesetwo publications disclose recycle of excess 1,3-propanediol into aterephthalic acid/1,3-propanediol paste mixer that is typicallyunheated. However, the stoichiometry set forth in examples 6, 7 and 8 ofboth indicates clearly that the recycled 1,3-propanediol did not resultfrom a steady state continuous process. Moreover, the process producedpoly(trimethylene terephthalate) with significant color, as suggested bythe use of cobalt compounds as color agents in examples 6 and 7.

These problems in recycle of 1,3-propanediol have resulted in reports(see, e.g., U.S. Pat. No. 6,657,044) that it is necessary to remove thesolid by-products from the recovered by-product 1,3-propanediol in orderto successfully recycle it. U.S. Pat. No. 6,657,044 teaches a processfor preparation of poly(trimethylene terephthalate) by esterification ofterephthalic acid or dimethyl terephthalate with 1,3-propanediol, whereexcess 1,3-propanediol is purified before recycle into the process. The1,3-propanediol stream is boiled and 1,3-propanediol is separated fromthe high boiling byproduct fraction consisting of solids andsemi-solids. The solids and semi-solids are heated in the presence of ametal catalyst which digests and converts the solid by-product to estersof terephthalic acid.

U.S. Pat. No. 6,245,879 discloses procedures for purification of acarbonyl-containing 1,3-propanediol stream for reuse in apoly(trimethylene terephthalate) process.

U.S. Pat. No. 6,703,478 and EP-B1245606 disclose a process forcontinuously producing an aromatic polyester comprising an aromaticdicarboxylic acid as the main dicarboxylic acid component and at leastone glycol selected from the group consisting of ethylene glycol,1,3-propanediol and 1,4-butanediol as the main glycol component throughan esterification or ester exchange reaction and a polycondensationreaction, wherein the distillate containing the above glycol from thepolycondensation reaction is subjected to at least flash distillation toremove low boiling substances before recycle to the esterification orester exchange reaction.

It would be highly advantageous to the continuous poly(trimethyleneterephthalate) polymerization process to be able to substantially reducethe amount of fouling due to precipitation of solids from the liquidby-product 1,3-propanediol, particularly in the precondensation stage.In addition, it would be advantageous to be able to recycle liquidby-product 1,3-propanediol into the process with minimal processing,while at the same time obtaining excellent quality poly(trimethyleneterephthalate) product.

SUMMARY OF THE INVENTION

This invention is directed to a continuous process for the production ofpoly(trimethylene terephthalate) comprising the steps of:

(a) continuously producing poly(trimethylene terephthalate) oligomerscomprising 1,3-trimethylene and terephthalate repeating units and havinga degree of polymerization of from about 1.9 to about 3.5 by (i) esterexchange reaction of dimethyl terephthalate with excess 1,3-propanediolat an elevated temperature or (ii) direct esterification reaction ofterephthalic acid with excess 1,3-propanediol at an elevatedtemperature;

(b) continuously precondensing the poly(trimethylene terephthalate)oligomers to form a poly(trimethylene terephthalate) prepolymer havingan intrinsic viscosity of at least about 0.23 dl/g and gaseousby-products comprising volatilized by-product 1,3-propanediol; and

(c) continuously polymerizing the poly(trimethylene terephthalate)prepolymer to form higher molecular weight poly(trimethyleneterephthalate) having an intrinsic viscosity of at least about 0.55 dl/gand additional gaseous by-products comprising volatilized by-product1,3-propanediol,

wherein:

(i) the gaseous by-products are condensed in at least one spraycondenser to form condensed by-product 1,3-propanediol, which is thencollected in at least one hotwell under conditions such that thetemperature of the condensed by-product 1,3-propanediol entering the atleast one hotwell is at about 50° C. or lower;

(ii) a portion of the condensed by-product 1,3-propanediol from thehotwell is cooled in at least one heat exchanger and then sprayed in theat least one spray condenser to condense the gaseous by-products; and

(iii) a portion of the condensed by-product 1,3-propanediol from thehotwell, without purification, is fed back into the ester exchange ordirect esterification reactions at one or more locations where thetemperature is about 150° C. or higher.

Preferably the condensed by-product 1,3-propanediol entering the atleast one hotwell is at about 45° C. or lower. Preferably the condensedby-product 1,3-propanediol entering the at least one hotwell is at leastabout 30° C., more preferably at least about 35° C.

The extent of fouling on pipes, heat exchanger walls and spray nozzlesin contact with the condensed by-product 1,3-propanediol due toprecipitation of solid by-products is less than that occurring with thesame process except wherein the temperature of the condensed by-product1,3-propanediol entering the same at least one hotwell is at least 55°C., preferably at about 55° C. In making this comparison, if one hotwellis operated at the temperature of the invention, comparison should bewith a system operating the same hotwell under these conditions, whereasif two or more hotwells are operated per the invention then thecomparison should be with the same hotwells being operated at thistemperature.

Preferably the additional gaseous by-products are condensed in at leastone spray condenser to form at least one stream of condensed by-product1,3-propanediol which is then collected in at least one hotwell andcooled in at least one heat exchanger under conditions such that thetemperature of the condensed by-product 1,3-propanediol entering the atleast one hotwell is about 50° C. or less. Preferably the condensedby-product 1,3-propanediol from the additional gaseous by-productsentering the at least one hotwell is at about 45° C. or lower.Preferably the condensed by-product 1,3-propanediol from the additionalgaseous by-products entering the at least one hotwell is at least about30° C., more preferably at least about 35° C.

Generally the condensed by-product 1,3-propanediol comprises1,3-propanediol and solid by-product comprising a mixture oftrimethylene terephthalate cyclic dimer and poly(trimethyleneterephthalate) oligomers.

Preferably the Hunter b color of the higher molecular weightpoly(trimethylene terephthalate) is below about 11.5.

In an alternative embodiment, the invention is directed to a continuousprocess for the production of poly(trimethylene terephthalate)comprising steps (a), (b) and (c) above, wherein:

(i) the gaseous by-products and the additional gaseous by-products arecondensed in at least two spray condensers to form condensed by-product1,3-propanediol, which is then collected in at least one hotwell underconditions such that the temperature of the condensed by-product1,3-propanediol entering the at least one hotwell is about 50° C. orlower;

(ii) a portion of the condensed by-product 1,3-propanediol is cooled inat least two heat exchangers and then sprayed in the at least two spraycondensers to condense the gaseous by-products and additional gaseousby-products; and

(iii) a portion of the condensed by-product 1,3-propanediol from thehotwell, without purification, is fed back into the ester exchange ordirect esterification reactions at one or more locations where thetemperature is about 150° C. or higher.

Preferably the gaseous by-products and the additional gaseousby-products are condensed in at least two spray condensers to form atleast two streams of condensed by-product 1,3-propanediol which are thencollected in at least one hotwell and cooled in at least two heatexchangers under conditions such that the temperature of the condensedby-product 1,3-propanediol entering the at least one hotwell is about50° C. or lower. Preferably the condensed by-product 1,3-propanediolentering the at least one hotwell is at about 45° C. or lower.Preferably the condensed by-product 1,3-propanediol entering the atleast one hotwell is at least about 30° C., more preferably at leastabout 35° C. Other preferences are described above and below.

Preferably (i) the gaseous by-products are condensed in at least onespray condenser to form at least one stream of condensed by-product1,3-propanediol which is then collected in at least one hotwell, (ii)the temperature of the condensed by-product 1,3-propanediol entering theat least one hotwell is at about 50° C. or lower, (iii) a portion of thecondensed by-product 1,3-propanediol is transferred (e.g., pumped) fromthe hotwell to at least one heat exchanger where it is cooled, and isthen sprayed in the at least one of the spray condensers to condense theby-product 1,3-propanediol, and (iv) at least 75 wt % of the condensedby-product 1,3-propanediol without purification is fed back into theester exchange or direct esterification reaction at one or morelocations where the temperature is about 150° C. or higher.

In one preferred embodiment, the ester exchange or direct esterificationreaction is carried out in one or more reaction vessels and the at leasta portion of the condensed by-product 1,3-propanediol withoutpurification is fed directly back into at least one of the one or morereaction vessels.

In another preferred embodiment, (i) the ester exchange or directesterification reaction is carried out in one or more reaction vessels,(ii) product methanol or water and carryover 1,3-propanediol is removedfrom the one or more reaction vessels as a vapor phase, (iii) the vaporphase is separated using a column into (A) a water or methanol phase and(B) a recovered 1,3-propanediol phase which is condensed into the baseof the column or a separate receiving vessel and then returned to theone or more reaction vessels, (iv) and the condensed by-product1,3-propanediol without purification is fed into the column, a receivingvessel at the base of the column, or the pipe(s) feeding the recoveredcondensed by-product 1,3-propanediol from the column into the reactionvessel, at a point where the temperature is about 150° C. or higher,preferably into the (I) vapor phase or (II) recovered 1,3-propanediolphase.

Preferably the gaseous by-products are condensed in at least one spraycondenser to form condensed by-product 1,3-propanediol comprising1,3-propanediol and by-product solids comprising trimethyleneterephthalate cyclic dimer and, optionally, poly(trimethyleneterephthalate), which is then collected in at least one hotwell andwherein a portion of the condensed by-product 1,3-propanediol is cooledin at least one heat exchanger and then sprayed in the at least onespray condenser, and further wherein the total amount of trimethyleneterephthalate cyclic dimer and poly(trimethylene terephthalate) in thecondensed by-product 1,3-propanediol is raised at least about 0.2 weight% based on the weight of condensed by-product 1,3-propanediol.

Thus, the invention provides a continuous poly(trimethyleneterephthalate) polymerization process wherein the amount of fouling dueto precipitation of solids from the liquid by-product 1,3-propanediol,particularly in the precondensation stage, is substantially reduced.According to a preferred embodiment, it is possible to recycle liquidby-product 1,3-propanediol into the process without purification of therecycle stream, while at the same time obtaining excellent qualitypoly(trimethylene terephthalate).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of an apparatus used to assess theextent of precipitation of solid by-products during the process of theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

All publications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety. Unlessotherwise defined, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which this invention belongs. In case of conflict, the presentspecification, including definitions, will control.

Except where expressly noted, trademarks are shown in upper case.

Although methods and materials similar or equivalent to those describedherein can be used in the practice or testing of the present invention,suitable methods and materials are described herein.

Unless stated otherwise, all percentages, parts, ratios, etc., are byweight.

When an amount, concentration, or other value or parameter is given aseither a range, preferred range or a list of upper preferable values andlower preferable values, this is to be understood as specificallydisclosing all ranges formed from any pair of any upper range limit orpreferred value and any lower range limit or preferred value, regardlessof whether ranges are separately disclosed. Where a range of numericalvalues is recited herein, unless otherwise stated, the range is intendedto include the endpoints thereof, and all integers and fractions withinthe range. It is not intended that the scope of the invention be limitedto the specific values recited when defining a range.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having” or any other variation thereof, areintended to cover a non-exclusive inclusion. For example, a process,method, article, or apparatus that comprises a list of elements is notnecessarily limited to only those elements but may include otherelements not expressly listed or inherent to such process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive or and not to an exclusive or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B are true (orpresent).

Use of “a” or “an” are employed to describe elements and components ofthe invention. This is done merely for convenience and to give a generalsense of the invention. This description should be read to include oneor at least one and the singular also includes the plural unless it isobvious that it is meant otherwise.

The materials, methods, and examples herein are illustrative only and,except as specifically stated, are not intended to be limiting.

The process of the present invention is an improved continuous processfor the production of poly(trimethylene terephthalate). The processcomprises the steps: (a) continuously producing poly(trimethyleneterephthalate) oligomers comprising 1,3-trimethylene and terephthalaterepeating units and having a degree of polymerization of from about 1.9to about 3.5: (b) continuously precondensing the oligomers to form apoly(trimethylene terephthalate) prepolymer; (c) continuouslypolycondensing the poly(trimethylene terephthalate) prepolymer to formhigher molecular weight poly(trimethylene terephthalate) having anintrinsic viscosity of at least about 0.55 dl/g.

The feed material for precondensation may be produced either by esterexchange from dimethyl terephthalate and 1,3-propanediol or by directesterification from terephthalic acid and 1,3-propanediol. Bothprocesses yield bis-3-hydroxypropyl terephthalate (referred to as“monomer”) and low molecular weight polyesters of 1,3-propanediol andterephthalic acid having an average degree of polymerization of 1.9 toabout 3.5 (referred to as “poly(trimethylene terephthalate) oligomers”).

A preferred process for direct esterification of terephthalic acid and1,3-propanediol is described in U.S. Pat. No. 6,887,953. Generally thedirect esterification or ester exchange is carried out at temperaturesof from about 235° C. to about 255° C.

Other processes for direct esterification and ester exchange are known,for instance as described in U.S. Pat. No. 6,277,947, U.S. Pat. No.6,326,456 and U.S. Pat. No. 6,353,062. Direct esterification or esterexchange can be carried out in one or more steps (or vessels), such asusing one vessel or multiples vessels (e.g., two or three) in series. Ina two-step esterification process, by-product 1,3-propanediol can beadded to one or both steps, but is preferably added to the first step.

The feed material for the esterification or ester exchange can containfrom about 0.01 to about 0.2 mole %, based on the total number of molesof 1,3-propanediol and diacid or diester (e.g., terephthalic acid ordimethyl terephthalate), of polyfunctional reactant containing three ormore carboxylic acid type groups or hydroxy groups, such as described inUS2006-013573A1. The polyfunctional repeat units can be present in thesame or different amounts, and may be the same or different, in eachcomponent.

If present, the polyfunctional reactant is preferably selected from thegroup consisting of polycarboxylic acid having at least three carboxylgroups and polyols having at least three hydroxyl groups, or mixturesthereof. Preferably the polyfunctional reactant is polycarboxylic acidhaving 3 to 4 carboxyl groups, more preferably having 3 carboxyl groups.Preferably the polyfunctional reactant is polyol having 3-4 hydroxylgroups, more preferably having 3 hydroxyl groups. In one embodiment thepolyfunctional reactant comprises polycarboxylic acid selected from thegroup consisting of trimesic acid, pyromellitic acid, pyromelliticdianhydride, benzophenone tetracarboxylic acid anhydride, trimelliticacid anhydride, benzenetetracarboxylic acid anhydride, hemimelliticacid, trimellitic acid, 1,1,2,2, ethanetetracarboxylic acid,1,2,2-ethanetricarboxylic acid, 1,3,5-pentanetricarboxylic acid,1,2,3,4-cyclopentanecarboxylic acid, and mixtures thereof. In anotherembodiment the polyfunctional reactant comprises polyol selected fromthe group consisting of glycerine, pentaerythritol,2-(hydroxymethyl)-1,3-propanediol, trimethylolpropane, and mixturesthereof. Most preferably the polyfunctional reactant comprises trimesicacid.

Trifunctional comonomers, for example trimellitic acid, can also beincorporated for viscosity control.

Whether the monomer/oligomer mixture described above is produced bydirect esterification from terephthalic acid or ester exchange fromdimethyl terephthalate, a catalyst is added prior to the esterificationor transesterification reaction. Catalysts useful in the ester exchangeprocess include organic and inorganic compounds of titanium, lanthanum,and zinc. Titanium catalysts, such as tetraisopropyl titanate and tetran-butyl titanate are preferred and are added to the 1,3-propanediolpreferably in an amount sufficient to yield from about 20 to about 200ppm, most preferably from about 50 to about 150 ppm of titanium byweight based on the weight of the finished polymer. These levels producerelatively low levels of unreacted dimethyl terephthalate in the esterexchange reaction (less than 5% by weight based on the total weight ofthe exit stream from the ester exchange) and give reasonable reactionrates in the precondensation and polycondensation steps.

Catalysts useful in the direct esterification process includeorgano-titanium and organo-tin compounds, which are added to the1,3-propanediol in an amount sufficient to yield at least about 20 ppmof titanium or at least about 50 ppm of tin, respectively, by weightbased on the finished polymer.

Additional catalyst may be added to the monomer/oligomer mixture afterthe ester exchange or direct esterification reaction and prior toprecondensation.

Whether the monomer/oligomer mixture is produced by directesterification from terephthalic acid or ester exchange from dimethylterephthalate, the degree of polymerization is preferably from about 1.9to about 3.5.

In a preferred embodiment of the invention, the monomer/oligomer mixtureis pumped from the ester exchange or direct esterification reactionstage to a precondensation stage by means of a temperature-controlledfeed line equipped with pumps. In the feed lines, the monomer/oligomermixture is maintained at a temperature of about 215° C. to about 250° C.

Precondensation can be carried out using one or more steps (or vessels),such as using one vessel or multiples vessels (e.g., two or three) inseries. Examples of suitable processes that can be modified to carry outthis invention are described in U.S. Pat. No. 6,277,947, U.S. Pat. No.6,326,456, U.S. Pat. No. 6,353,062, U.S. Pat. No. 6,538,076,US2003-0220465A1 and US2005-0165178A1.

The volatilized by-product 1,3-propanediol and any other volatileby-products from the precondensation are removed through a vapor lineconnected to a vacuum source as a stream of gaseous by-products, andthen condensed.

The by-product 1,3-propanediol vapors from precondensation typicallycontain other reaction by-products such as acrolein and allyl alcohol.It is desirable that the production of by-products such as acrolein andallyl alcohol be minimized because both of these compounds are highlytoxic and cause irritation to the eyes and mucous membranes.

Intrinsic viscosity is an indicator of molecular weight. Intrinsicviscosity, often referred to as “IV,” as discussed herein is determinedin a solvent consisting of 50 wt % trifluoroacetic acid, 50 wt %dichloromethane (“TFA/CH₂Cl₂”) employing a VISCOTEK FORCED FLOWVISCOMETER MODEL Y-900 to measure the IV of polymer dissolved at aconcentration of 0.4% (wt/vol) in 50/50 wt % TFA/CH₂Cl₂ at 19° C. Thepoly(trimethylene terephthalate) prepolymer from the prepolymerizationpreferably has an intrinsic viscosity of at least about 0.23 dl/g andpreferably up to about 0.35 dl/g, more preferably from about 0.25 toabout 0.30 dl/g.

The prepolymer product is fed to a final polymerization orpolycondensation stage. The major purpose of the polycondensation is toincrease the molecular chain length or viscosity of the polymer. This isaccomplished by using heat, agitation, vacuum and catalyst. It isdesirable that the molecular weight of the finished polymer bemaximized, so that further processing, e.g., solid state polymerization,can be avoided prior to fiber spinning or other forming operation.

Polycondensation can be carried out using one or more steps (orvessels), such as using one vessel or multiples vessels (e.g., two orthree) in series. Examples of suitable processes that can be modified tocarry out this invention are described in U.S. Pat. No. 6,277,947, U.S.Pat. No. 6,326,456, U.S. Pat. No. 6,353,062, U.S. Pat. No. 6,538,076,US2003-0220465A1 and US2005-0165178A1. The temperature of the liquidreactants in the polycondensation stage is preferably maintained atabout 245° C. to about 265° C., more preferably about 255° C. to about265° C. The pressure is maintained at about 0.5 to about 3.0 mm Hg (66to 399 Pa). The viscosity of the finished polymer may be controlled byadjusting polycondensation pressure or other variables. The residence orhold-up time in the polycondensation stage is typically about 1 to about3 hours. The intrinsic viscosity of the higher molecular weightpoly(trimethylene terephthalate) after polycondensation is at leastabout 0.55, preferably at least about 0.85, more preferably at leastabout 0.91, more preferably at least about 0.96, and most preferably atleast about 1.0 dl/g. Intrinsic viscosity can be as high as about 1.2 ormore dl/g, and is typically up to about 1.15 or about 1.05 dl/g,depending on the desired end-use.

1,3-Propanediol and other gaseous by-products are produced duringpolycondensation as a stream of gaseous by-products and then condensed.One method for condensing the 1,3-propanediol vapors is by means of aspray condenser similar to that described above for condensing1,3-propanediol vapors from the precondensation.

The stream of condensed by-product 1,3-propanediol produced duringpolycondensation is collected in a hotwell.

According to a preferred embodiment of the invention, at least a portionof the streams of condensed by-product 1,3-propanediol in the hotwells,preferably at least about 75 wt % of the byproduct 1,3-propanediol, maybe fed back into the ester exchange or direct esterification reactionswithout purification at a location where the temperature is greater thanabout 150° C. By the phrase “without purification” it is meant thatthere is no chemical treatment or physical separation, e.g.,distillation or solids or volatiles removal, carried out on thecondensed by-product 1,3-propanediol.

By the phrase “fed back into the ester exchange or direct esterificationreactions” it is meant that the condensed by-product 1,3-propanediol (a)is fed directly into the reaction vessel, (b) is fed to the vapor phasecoming out of the esterifier (i.e., to the column used to separate thewater or methanol from the 1,3-propanediol or the base of the column),or (c) is fed to any line or small receiving vessel connecting thecolumn and a reaction vessel used for the esterification or esterexchange, such as a line feeding the material exiting the column into areaction vessel. Specifically excluded is feeding the condensedby-product 1,3-propanediol to the raw materials (e.g., fresh1,3-propanediol) or the paste of raw materials that enters the firstreactor.

Thus, according to this embodiment of the invention the gaseousby-products and the additional gaseous by-products are condensed in atleast two spray condensers, at least one for precondensing stage and atleast one for polycondensing stage, to form at least two streams ofcondensed by-product 1,3-propanediol which are then collected in atleast one hotwell. Preferably at least one hotwell is used for theprecondensation stage and at least one hotwell is used for thepolycondensing stage. However, the streams of condensed by-product1,3-propanediol from the precondensation stage and condensed by-product1,3-propanediol from the polycondensing stage can be combined aftercondensing and collected in a single hotwell. At least a portion of thecondensed by-product 1,3-propanediol from the precondensation stage isfed back into the ester exchange or direct esterification reactions. Atleast a portion of the condensed by-product 1,3-propanediol from thepolycondensation stage may also be fed back into the ester exchange ordirect esterification reactions, directly or after combination with thecondensed by-product 1,3-propanediol from the precondensation stage.

The finished polymer may be pelletized or fed directly to a formingoperation, such as fiber spinning, film formation or molding operation.Fibers made from the poly(trimethylene terephthalate) produced by theprocess of the invention have properties which make them useful invarious textile applications, including the manufacture of carpet orapparel.

Various additives also may be used in the process of the invention.These may include color inhibitors, such as phosphoric acid,delustrants, such as titanium dioxide, dyeability modifiers, pigmentsand whiteners. If separate ester exchange and polymerization catalystsare used, phosphoric acid or other color inhibitors may be added tominimize or prevent the color forming property of the ester exchangecatalyst. One advantage of the process of this invention is that it isgenerally not necessary to use color inhibitors or stabilizers, such asphosphoric acid, organophosphites, phenols, amines, and whiteners, suchas those used in order to reduce acrolein and allyl alcohol or toimprove polymer color.

As has been pointed out in U.S. Pat. No. 6,657,044 and U.S. Pat. No.6,245,879, the streams of condensed by-product 1,3-propanediol generallycontain small amounts of carbonyl compounds such as acrolein as well assmall amounts of solid and semi-solid by-products, hereinaftercollectively described as “solid by-products”. The solid by-productshave been characterized as comprising trimethylene terephthalate cyclicdimer and poly(trimethylene terephthalate) oligomers. Moreover, if thestarting material for the process includes dimethyl terephthalate, theremay even be small amounts of dimethyl terephthalate found in therecovered 1,3-propanediol.

U.S. Pat. No. 6,657,044 and U.S. Pat. No. 6,245,879 further indicatethat in order to obtain high quality poly(trimethylene terephthalate)when recycling the condensed by-product 1,3-propanediol, it is necessaryto purify the condensed by-product 1,3-propanediol to remove thecarbonyl compounds and solid by-products. However, it has now been foundthat the preferred embodiment process of the invention allows thecondensed by-product 1,3-propanediol to be recycled to theesterification or ester exchange reactions without purification andstill produce poly(trimethylene terephthalate) with quality suitable foruse in the conventional end-use applications such as fibers, films andmolding applications. Indeed it has been found that both the viscosityand color characteristics of the poly(trimethylene terephthalate)product prepared using recycled 1,3-propanediol from the process of theinvention without purification, are essentially the same as thatprepared in the same way but without recycling the 1,3-propanediol.

It has been found that during long term operation of a continuousprocess for preparation of poly(trimethylene terephthalate) by theprocesses disclosed in U.S. Pat. No. 6,538,076 and U.S. Pat. No.6,353,062, some precipitation of the solid by-products may occur. Asthese precipitates build up over time on the pipes, heat exchanger wallsand spray nozzles, etc. in contact with the condensed by-product1,3-propanediol, they may cause fouling, which results in lower flowrates and eventual deleterious spray condenser operation with subsequentloss of vacuum. This problem is most notable in the precondensationstage of the process. The result is increased downtime due to the needto shut down in order to remove the precipitated solids.

The process of the invention provides a method to minimize or eliminatethe deleterious precipitation of by-product solids, as well as apreferred embodiment encompassing a second method to minimize oreliminate the deleterious precipitation of by-product solids. Accordingto the invention, it has been found that in spite of the general highersolubility of solids at higher temperatures, precipitation and foulingin this process is minimized if the condensed by-product 1,3-propanediolis collected in a hotwell and cooled in a heat exchanger underconditions such that the temperature of the condensed by-product1,3-propanediol entering the hotwell is no higher than about 50° C.,preferably 35-45° C. This has been confirmed in operations where it hasbeen shown that run life can be extended by several months due to lowerfouling rates when this process improvement is utilized.

In the preferred method, it has been found that fouling downstream ofthe heat exchanger is minimized if the by-product solids level,specifically the amount of trimethylene terephthalate cyclic dimer andpoly(trimethylene terephthalate), in the condensed by-product1,3-propanediol is raised, and is maintained at a level preferably from1 to about 10 wt %, based on the weight of condensed by-product1,3-propanediol. The specific amount of trimethylene terephthalatecyclic dimer and poly(trimethylene terephthalate) that should be usedwill vary depending upon the starting materials and process conditions.For instance, the presence of dimethyl terephthalate (“DMT”) increasesfouling and higher levels of trimethylene terephthalate cyclic dimer andpoly(trimethylene terephthalate) seem to be necessary when DMT is used.DMT is preferably present in the condensed by-product 1,3-propanediolfrom the precondensation stage (and also preferably from thepolycondensation stage) at levels of about 0.3 wt % or less, morepreferably about 0.2 wt % or less, and most preferably about 0.1 wt % orless, with 0% being most preferred (e.g., when terephthalic acid isused). Typically, the preferred total amount of trimethyleneterephthalate cyclic dimer and poly(trimethylene terephthalate) in thecondensed by-product 1,3-propanediol is raised is at least about 0.2 toabout 7 wt %, based on the weight of condensed by-product1,3-propanediol. Under some circumstances, at least about 0.3, at leastabout 0.5, and even higher amounts such as at least about 0.7, or atleast about 1 wt %, can be preferred. In addition, raising it less, suchas about 6 wt % or less, about 5 wt % or less, about 3 wt % or less,about 2 wt % or less, and 1.5 wt % or less, can be preferred.

In one way of practicing this second method, preferably highsolids-containing condensed by-product 1,3-propanediol from thepolycondensation hotwell (which generally contains the highest level ofsolids) can be transferred back to the precondensation hotwell(s)(consecutively from the last precondensation hotwell to the firstprecondensation hotwell) in order to raise solids levels in theprecondensation hotwells. In this regard, it should be noted that about10 to about 30 times as much gaseous by-product is produced duringprecondensation than during polycondensation, so that proportionallysmall amounts of condensed by-product from the polycondensation stagecan be added to the condensed by-product from the precondensation stage.This can be via direct addition during the process or by storing some orall of the condensed by-product from the polycondensation and,optionally, treating it prior to use. A second approach, preferablyinvolves filtering and withdrawing a portion of the 1,3-propanediol outof the recirculating mixture of condensed by-product 1,3-propanediol andtrimethylene terephthalate cyclic dimer to raise the solids content ofthe resulting recirculating 1,3-propanediol. In a third way, finelyground poly(trimethylene terephthalate) and/or trimethyleneterephthalate cyclic dimer are added to the recirculating condensedby-product 1,3-propanediol.

EXAMPLES

The following examples are presented for the purpose of illustrating theinvention and are not intended to be limiting. All parts, percentages,etc., are by weight unless otherwise indicated.

Measurement of polymer L, a, and b colors was performed using aHUNTER-LAB LABSCAN XE with DP-9000 system. The DP-9000 performsintegration of reflectance values over the visible spectrum to arrive atCIE tristimulus X, Y and Z values as outlined in publication CIE 15.2and ASTM Method E308. The tristimulus X, Y and Z values are used tocalculate Hunter L, a, and b values.

Procedures for Examples 1-8 and Comparative Examples 1 and 2

Examples 1-8 and Comparative Examples 1 and 2 are concerned withdetermining the amount of precipitation of polymerization by-products,chiefly trimethylene terephthalate cyclic dimer in recirculating1,3-propanediol. The apparatus used for these examples is describedbelow.

The apparatus was a temperature-controlled circulating bath asillustrated in FIG. 1. The bath 1 contained approximately 3.5 liters of1,3-propanediol mixed with 1 wt % trimethylene terephthalate cyclicdimer. For Examples 1 and 2 and Comparative Example 1, the exit of thecirculating bath was attached to a straight 0.25 inch internal diameterglass tube 2 of a water-cooled heat exchanger. Cold water from a secondcirculating bath 6 was passed through the jacket 3 along the outside ofthe glass tube. The heated mixture of 1,3-propanediol and trimethyleneterephthalate cyclic dimer was circulated through the inner glass tube 2at an initial flow rate of approximately 550 cc/min. Thermocouples 4 and5 were mounted at the inlet and outlet respectively of the glass tube.After 24 hours continuous operation, the inner glass tube was removedand rinsed with water. After rinsing, a layer of white precipitateadhered to the inside of the inner glass tube.

In operation, the inlet and outlet 1,3-propanediol/ trimethyleneterephthalate cyclic dimer mixture temperatures were monitored. Asprecipitation and fouling occurred to the point of restricting flow, agradual decrease in flow resulted in increased cooling or a lower outlettemperature. Consequently, the difference in outlet temperature betweenthe beginning and the end of the test was taken as a measure of theamount of precipitation.

For Examples 3-8 and Comparative Example 2, the exit of the circulatingbath was attached to a 12.5 inch long by 5/32 inch inner diameter glasstube 2 that was inserted inside of a 9.38 inch long by 1 inch innerdiameter glass tube 3. The heated mixture of 1,3-propanediol andtrimethylene terephthalate cyclic dimer was circulated through the innerglass tube 2 at approximately 340 cc/min, and cooling water from asecond circulating bath 6 was passed through the outer glass tube 3.

Examples 1 and 2 and Comparative Example 1

In Comparative Example 1, the mixture of 1,3-propanediol andtrimethylene terephthalate cyclic dimer was circulated through the innerglass tube at an inlet temperature of 55.2° C., and in Examples 1 and 2at 45.3° C. and 39.7° C., respectively. The results are in Table 1.

TABLE 1 Comp. Exp. 1 Exp. 1 Exp. 2 Wt % Cyclic Dimer 1.0 1.0 1.0 InletTemp. (° C.) 55.2 45.3 39.7 Initial Outlet Temp. (° C.) 53.3 43.7 38.2Test Duration (Hrs.) 24 24 24 Final Outlet Temp. (° C.) 52.3 43.7 38.2Outlet Temp. Drop (° C.) 1 0 0

In Comparative Example 1, where the circulation temperature was above50° C. entering the heat exchanger, precipitation and fouling occurredto the point of restricting flow as evident from the increased coolingafter 24 hours, i.e. lower outlet temperature. In contrast, in Examples1 and 2, where the circulation temperature was below about 50° C., therewas essentially no decrease in outlet temperature, which was indicativeof no, or minimal fouling. This indicated that maintenance of thecirculating condensed by-product 1,3-propanediol at temperatures nohigher than about 50° C. minimized the amount of fouling caused byprecipitation of trimethylene terephthalate cyclic dimer.

Examples 3-6

These examples were carried out with an apparatus utilizing a 5/32 inchoutlet tube instead of the 0.25 inch outlet tube used in Examples 1 and2. It was therefore expected that the effect of fouling on flowrestriction should be greater in these examples than in the previousones. The results are in Table 2.

TABLE 2 Exp. 3 Exp. 4 Exp. 5 Exp. 6 Wt. % Cyclic Dimer 1.0 1.0 1.5 5.0Inlet Temp. (° C.) 49.9 45.0 50.0 45.5 Initial Outlet Temp. (° C.) 47.042.0 46.0 41.5 Test Duration (Hrs.) 24 24 24 24 Final Outlet Temp. (°C.) 45.5 41.1 45.4 40.7 Outlet Temp. Drop (° C.) 1.5 0.9 0.6 0.8

As expected, the effect of the lower diameter tube in these examples wasto somewhat increase the observed outlet temperature drop. However,comparison of the results of Examples 4, 5, and 6 to those of Example 3indicate that when the circulation temperature was less than about 50°C., the amount of precipitation/fouling was less than it was at about50° C. as evident from the temperature drops. Thus, in addition todemonstrating the advantages of use of a lower temperature, it wasunexpectedly discovered that by increasing the content of trimethyleneterephthalate cyclic dimer the fouling decreased.

Examples 7 and 8 and Comparative Example 2

These examples illustrate the effect on precipitation and fouling ofincreasing the solids level of trimethylene terephthalate cyclic dimerand poly(trimethylene terephthalate) as in the preferred embodiment ofthis invention. The examples were carried out using the same apparatusdescribed above for Examples 3, 4, 5, and 6.

In all of these examples a low level of dimethyl terephthalate (DMT) wasincluded in the 1,3-propanediol mixture to simulate the situation wheredimethyl terephthalate is used as the starting material for preparationof poly(trimethylene terephthalate). The poly(trimethyleneterephthalate) used in these examples had an intrinsic viscosity of 1.02dl/g and was cryoground and screen filtered to above 80 mesh in Example8 and to between 60 and 80 mesh in Example 7.

The results are in Table 3.

TABLE 3 Comp. Exp. 2 Exp. 7 Exp. 8 Wt. % Cyclic Dimer 1.0 1.0 1.0 Wt. %DMT 0.2 0.2 0.2 Wt. % PTT 0 1.0 1.0 PTT Mesh Size — 60-80 >80 InletTemp. (° C.) 50.0 50.4 50.5 Initial Outlet Temp. (° C.) 47.0 47.7 47.0Test Duration (Hrs.) 24 90 65 Final Outlet Temp. (° C.) 43.5 43.5 43.7Outlet Temp. Drop (° C.) 3.5 4.0 2.3

Comparison of Example 3 with comparative example 2 shows that theaddition of small amounts of dimethyl terephthalate accelerates fouling.Thus, the inlet temperature of 50° C., which was acceptable without thepresence of dimethyl terephthalate (Example 3) is less acceptable in thepresence of dimethyl terephthalate (Comparative Example 2) and a lowerinlet temperature or higher solids level, i.e., the trimethyleneterephthalate cyclic dimer and poly(trimethylene terephthalate) levels,should be maintained when dimethyl terephthalate is present. However,the results of Examples 7 and 8 demonstrated that by increasing thesolids level, particularly the trimethylene terephthalate cyclic dimerand poly(trimethylene terephthalate) levels, in the circulating1,3-propanediol by addition of 1 wt % poly(trimethylene terephthalate)it was possible to reduce the level of precipitation/fouling as measuredby outlet temperature drop. It should be noted that in Example 8,recirculation was carried out for 65 hours as compared to only 24 hoursfor Comparative Example 2, but in spite of this resulted in a lowerlevel of fouling. In the case of Example 7, recirculation was carriedout for 90 hours and resulted in about the same level of precipitationas observed in Comparative Example 2 after 24 hours.

Example 9

This example demonstrates the advantages of a preferred embodiment ofthe invention and shows preparation of high quality poly(trimethyleneterephthalate) in a continuous process where the condensed by-product1,3-propanediol and other by-products are recycled back to theesterification reaction without purification.

A self-circulating esterifier designed as described in U.S. Pat. No.3,927,982, was operated at about 245° C. and a process pressure ofbetween 4 and 5 psig (129 to 136 kPa). Fresh 1,3-propanediol wascontinuously loaded into a 500 lb (227 kg) capacity feed tank from whichit was fed to make paste. A paste containing fresh 1,3-propanediol andterephthalic acid at a mole ratio of about 1.5 (35.5 kg/h terephthalicacid and 24.4 kg/h 1,3-propanediol), and TYZOR® TPT catalyst at a levelof 33 ppm Ti (relative to final polymer) was continually injected intothe esterifier at a polymer production rate of 44.1 kg/h (97 lb/h).Water and 1,3-propanediol vapors were continually extracted into adistillation column where the water and other by-products were separatedfrom 1,3-propanediol. The 1,3-propanediol that condensed from thedistillation column was collected into a heated esterifier condensatereceiver that was maintained at a temperature of 165° C. or higher. The1,3-propanediol in the receiver was returned back to the esterifier tomaintain an oligomer degree of polymerization of about 3.0 as describedin U.S. Pat. No. 6,887,953. Any excess 1,3-propanediol in the receiver,above that needed to maintain a degree of about 3.0, was recycled backto the 1,3-propanediol feed tank where it was mixed with fresh1,3-propanediol and then fed to make paste. Oligomer from the esterifierwas continually withdrawn and an additional 33 ppm Ti (relative to finalpolymer and in the form of TYZOR® TPT catalyst) and 34.5 mL/min of 20 wt% TiO₂ in 1,3-propanediol was injected into the oligomer before it waspassed through two precondensation vessels (in series) and apolycondensation vessel. Processing of the oligomer was accomplishedafter the method described in U.S. Pat. No. 6,538,076, to producepoly(trimethylene terephthalate) with intrinsic viscosities (IV) between0.90 and 0.94 dl/g.

1,3-Propanediol (about 8.1 kg/h) and other by-products were continuouslyvaporized and removed from the precondensation and polycondensationvessels. Vapors from the two precondensation vessels were condensed inspray condensers and collected in a precondensation hotwell. Vapor fromthe polycondensation vessel was condensed and collected in an adjacentpolycondensation hotwell. Liquid, comprised mostly of 1,3-propanediol,overflowed from the precondensation hotwell into the polycondensationhotwell. Solids (trimethylene terephthalate cyclic dimer andpoly(trimethylene terephthalate) in the polycondensation hotwell weremeasured at levels between 0.80 and 2.0 wt %.

After establishing stable polymer production, 1,3-propanediol from thepolycondensation hotwell was recycled at a rate of 100 mL/min (about 6.3kg/h) into the heated esterifier condensate receiver, which correspondedto a recycle rate of approximately 77.5%. This mode of recycle wasmaintained for over 6 days. Liquid in the heated esterifier condensatereceiver remained clear throughout the demonstration, indicating thatany solids in the condensed by-product 1,3-propanediol were able todissolve.

Using only fresh 1,3-propanediol for making paste, polymer L and bcolors were measured to be approximately 83.2 and 6.5, respectively.After beginning recycle, polymer L and b colors changed only slightly to82.1 and 7.1, respectively. The direct recycle of 1,3-propanediol fromthe precondensation and finisher hotwells in this manner thus providedan effective method for recycling 1,3-propanediol to make high qualitypolymer without purification of the recycled 1,3-propanediol oradditional handling and additives, as recommended in literature.

The forgoing disclosure of the embodiments of the present invention hasbeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formsdisclosed. Many variations and modifications of the embodimentsdescribed herein will be obvious to one of ordinary skill in the art inlight of the disclosure.

1. A continuous process for the production of poly(trimethyleneterephthalate) comprising the steps of: (a) continuously producingpoly(trimethylene terephthalate) oligomers comprising 1,3-trimethyleneand terephthalate repeating units and having a degree of polymerizationof from about 1.9 to about 3.5 by (i) ester exchange reaction ofdimethyl terephthalate with excess 1,3-propanediol at an elevatedtemperature or (ii) direct esterification reaction of terephthalic acidwith excess 1,3-propanediol at an elevated temperature; (b) continuouslyprecondensing the poly(trimethylene terephthalate) oligomers to form apoly(trimethylene terephthalate) prepolymer having an intrinsicviscosity of at least about 0.23 and gaseous by-products comprisingvolatilized by-product 1,3-propanediol; and (c) continuouslypolymerizing the poly(trimethylene terephthalate) prepolymer to formhigher molecular weight poly(trimethylene terephthalate) having anintrinsic viscosity of at least about 0.55 dl/g and additional gaseousby-products comprising volatilized by-product 1,3-propanediol, wherein:(i) the gaseous by-products are condensed in at least one spraycondenser to form condensed by-product 1,3-propanediol, which is thencollected in at least one hotwell under conditions such that thetemperature of the condensed by-product 1,3-propanediol entering the atleast one hotwell is at about 50° C. or lower; (ii) a portion of thecondensed by-product 1,3-propanediol from the hotwell is cooled in atleast one heat exchanger and then sprayed in the at least one spraycondenser to condense the gaseous by-products; and (iii) a portion ofthe condensed by-product 1,3-propanediol from the hotwell, withoutpurification, is fed back into the ester exchange or directesterification reactions at one or more locations where the temperatureis about 150° C. or higher.
 2. The process of claim 1, wherein thecondensed by-product 1,3-propanediol entering the at least one hotwellis at about 30° C. to about 45° C.
 3. The process of claim 1, whereinthe additional gaseous by-products are condensed in at least one spraycondenser to form at least one stream of condensed by-product1,3-propanediol which is then collected in at least one hotwell andcooled under conditions such that the temperature of the condensedadditional by-product 1,3-propanediol entering the at least one hotwellis about 50° C. or lower; and (ii) a portion of the condensed additionalby-product 1,3-propanediol from the hotwell is cooled in at least oneheat exchanger and then sprayed in the at least one spray condenser tocondense the gaseous by-products.
 4. The process of claim 3, wherein thecondensed by-product 1,3-propanediol from the additional gaseousby-products entering the at least one hotwell is at about 35° C. toabout 45° C.
 5. The process of claim 3, wherein a portion of thecondensed additional by-product 1,3-propanediol from the hotwell,without purification, is fed back into the ester exchange or directesterification reactions at one or more locations where the temperatureis about 150° C. or higher.
 6. The process of claim 1, wherein thecondensed by-product 1,3-propanediol comprises 1,3-propanediol and solidby-product comprising a mixture of trimethylene terephthalate cyclicdimer and poly(trimethylene terephthalate) oligomers.
 7. The process ofclaim 1, wherein (i) the gaseous by-products and the additional gaseousby-products are condensed in at least one spray condenser for theprecondensing stage (b) to form a condensed by-product 1,3-propanediol,and at least one spray condenser for the polycondensation stage (c) toform a condensed additional by-product 1,3-propanediol, which is thencollected in at least one hotwell under conditions such that thetemperature of the condensed by-product 1,3-propanediol and thecondensed additional by-product 1,3-propanediol entering the at leastone hotwell is about 50° C. or lower; (ii) wherein a portion of thecondensed by-product 1,3-propanediol and the condensed additionalby-product 1,3-propanediol is cooled in at least two heat exchangers andthen sprayed in the at least two spray condensers to condense thegaseous by-products and additional gaseous by-products; and (iii) aportion of the condensed by-product 1,3-propanediol and the condensedadditional by-product 1,3-propanediol from the hotwell, withoutpurification, is fed back into the ester exchange or directesterification reactions at one or more locations where the temperatureis about 150° C. or higher.
 8. The process of claim 1, wherein a directesterification reaction is used in step (a).
 9. The process of claim 1,wherein an ester exchange reaction is used in step (a).
 10. The processof claim 3, wherein a direct esterification reaction is used in step(a).
 11. The process of claim 3, wherein an ester exchange reaction isused in step (a).
 12. The process of claim 5, wherein a directesterification reaction is used in step (a).
 13. The process of claim 5,wherein an ester exchange reaction is used in step (a).
 14. The processof claim 7, wherein a direct esterification reaction is used in step(a).
 15. The process of claim 7, wherein an ester exchange reaction isused in step (a).